1. Field of the Invention
This invention relates to a combustion state-determining system for internal combustion engines, which detects misfires occurring in the engine, to thereby determine a state of combustion of the engine, based on the state of occurrence of misfires.
2. Prior Art
There is conventionally known a misfire-determining system for internal combustion engines, as proposed, for example, by Japanese Provisional Patent Publication (Kokai) No. 3-253770, which detects misfires occurring in the engine and determines that the combustion state of the engine is degraded (the engine is in a misfiring state) when misfires have occurred in the engine at a predetermined rate or more over a predetermined number of rotations of the engine.
To determine a misfiring state which causes degradation of the combustion state, however, the above proposed conventional system has to count the number of times of misfire occurrence over 1000 rotations of the engine. As a result, it can be erroneously determined that the engine is in an abnormal combustion state (continuous misfiring state), e.g. when transient misfires have temporarily and concentratedly occurred (e.g. misfires due to smoldering of a spark plug, surging of exhaust gas recirculation, etc.) during 1000 engine rotations, wherein the misfiring rate over the entire time period of 1000 engine rotations is so low that the transient misfiring state does not cause degradation of exhaust emission characteristics of the engine. In addition, the above conventional system is only capable of carrying out a mere determination as to whether the engine is in an abnormal combustion state, over a relatively long time period, but incapable of determining kinds of abnormal combustion states of the engine.